Responsivity and noise of a wire-bonded CMOS micro-fluxgate sensor

This paper presents a dual-core (Vacquier-type) micro-fluxgate magnetic sensor fabricated on a silicon chip based on standard CMOS technology. The silicon chip is 2.5 mm × 2.5 mm in dimension, and the micro-fluxgate sensor occupies the area of 2.5 mm × 1.8 mm. The sensor consists of magnetic cores, planar pick-up coils, bottom excitation coils (CMOS Al interconnections) and upper excitation coils (wire-bonding Al wires). The micro-solenoid excitation coils consisting of aluminum bonding wires and CMOS metallic layers can generate out-of-phase excitation magnetic fields strong enough to saturate the two magnetic cores. It was found that the sensor's sensitivity can be optimized and the field noise spectral density can be minimized by adjusting the excitation current as well as the excitation frequency. The maximum responsivity of 8.1 V/T occurs at the second harmonic when the excitation frequency is 110 kHz; however, the minimum magnetic field noise is found to be 2.6 nT/√Hz at 1 Hz under 20-kHz excitation. In comparison with other miniature planar fluxgates with similar dimensions reported to date, our device has a relatively low field noise spectral density.